ASP.NET 由于采用了管道式设计，具有很好的扩展性。整个ASP.NET MVC应用框架就是通过扩展ASP.NET实现的。通过ASP.NET的管道设计，我们知道，ASP.NET的扩展点主要是体现在HttpModule和HttpHandler这两个核心组件之上，实际上整个ASP.NET MVC的框架就是通过自定义的HttoModule和HttpHandler建立起来的。
当然，要从整体上把握ASP.NET MVC 的工作机制，我们可以通过查看其源码或自己实现一个“迷你版”的ASP.NET MVC 来了解其运行原理。
RouteData
ASP.NET定义了一个全局的路由表，路由表中的每个路由对象报刊一个URL模板。目标Controller和Action的名称可以通过路由变量以占位符（比如“{controller}”和“{action}”）定义在URL模板中，也可以作为路有对象的默认值。对于每一个抵达的HTTP请求，ASP.NET MVC会便利路由表找到一个具有与当前请求URL模式相匹配的路由对象，并最终解析出以Controller和Action名称为核心的路由数据。
1: public class RouteData
2: {
3: /// <summary>
4: /// 变量列表
5: /// </summary>
6: public IDictionary<string, object> Values { get; private set; }
7:
8: /// <summary>
9: /// 其他来源的变量列表
10: /// </summary>
11: public IDictionary<string, object> DataTokens { get; private set; }
12:
13: public IRouteHandler RouteHandler { get; set; }
14:
15: public RouteBase Route { get; set; }
16:
17: public RouteData()
18: {
19: this.Values = new Dictionary<string, object>();
20: this.DataTokens = new Dictionary<string, object>();
21: this.DataTokens.Add("namespaces", new List<string>());
22: }
23:
24: /// <summary>
25: /// 获取控制器名称
26: /// </summary>
27: public string Controller
28: {
29: get
30: {
31: object controllerName = string.Empty;
32: this.Values.TryGetValue("controller", out controllerName);
33: return controllerName.ToString();
34: }
35: }
36:
37: /// <summary>
38: /// 获取方法名称
39: /// </summary>
40: public string ActionName
41: {
42: get
43: {
44: object actionName = string.Empty;
45: this.Values.TryGetValue("action", out actionName);
46: return actionName.ToString();
47: }
48: }
49: }
RouteData的RouteHandler属性类型为IRouteHandler接口，该接口具有一个GetHttpHandler方法，用于返回真正用于处理HTTP请求的HttpHandler对象。
1: public interface IRouteHandler
2: {
3: IHttpHandler GetHttpHandler(RequestContext requestContext);
4: }
IRouteHandler接口的GetHttpHandler方法接受一个类型为RequestContext的参数，RequestContext表示当前HTTP请求的上下文，其核心就是对当前HttpContext和RouteData的封装。
1: public class RequestContext
2: {
3: public virtual HttpContextBase HttpContext { get; set; }
4:
5: public virtual RouteData RouteData { get; set; }
6: }
Route和RouteTable
RouteData具有一个类型为RouteBase的Route属性，该属性表示生成路由数据对应的路由对象。RouteBase是一个抽象类，它包含一个GetRouteData的方法，该方法用于判断是否与当前请求相匹配。并在匹配的情况下返回用于封装路由数据的RoutData对象。该方法接受一个表示当前HTTP上下文的HttpContextBase对象，如果与当前请求不匹配，则返回null。
1: public abstract class RouteBase
2: {
3: public abstract RouteData GetRouteData(HttpContextBase httpContext);
4: }
ASP.NET MVC 提供的基于URL模板的路由机制是通过其子类Route实现的。
1: public class Route : RouteBase
2: {
3:
4: public IRouteHandler RouteHandler { get; set; }
5:
6: public string Url { get; set; }
7:
8: public IDictionary<string, object> DataTokens { get; set; }
9:
10: public Route()
11: {
12: this.DataTokens = new Dictionary<string, object>();
13: this.RouteHandler = new MvcRouteHandler();
14: }
15:
16: public override RouteData GetRouteData(System.Web.HttpContextBase httpContext)
17: {
18: IDictionary<string, object> variables;
19: if(this.Match(httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2),out variables))
20: {
21: RouteData routeData = new RouteData();
22: foreach (var item in variables)
23: {
24: routeData.Values.Add(item.Key, item.Value);
25: }
26: routeData.RouteHandler = this.RouteHandler;
27: return routeData;
28: }
29: return null;
30: }
31:
32: public bool Match(string requestUrl, out IDictionary<string, object> variables)
33: {
34: variables = new Dictionary<string, object>();
35: string[] strArray1 = requestUrl.Split('/');
36: string[] strArray2 = this.Url.Split('/');
37: if (strArray1.Length != strArray2.Length)
38: {
39: return false;
40: }
41: for (int i = 0; i < strArray2.Length; i++)
42: {
43: if (strArray2[i].StartsWith("{") && strArray2[i].EndsWith("}"))
44: {
45: variables.Add(strArray2[i].Trim("{}".ToCharArray()), strArray1[i]);
46: }
47: else
48: {
49: if(string.Compare(strArray1[i],strArray2[i],true)!=0)
50: {
51: return false;
52: }
53: }
54: }
55: return true;
56: }
57: }
由于同一个Web应用可以采用多种不同的URL模式，所欲需要注册多个继承自RouteBase的路由对象，多个路由对象组成一个路由表。
1: public class RouteTable
2: {
3: public static RouteDictionary Routes { get; private set; }
4:
5: static RouteTable()
6: {
7: Routes = new RouteDictionary();
8: },
9: }
RouteDictionary表示一个具名的路由对象列表。这里我们让它继承自Dictionary<string,RouteBase>，其中key表示路由对象的注册名称。在System.Web.Routing中，它实际上是继承自RouteCollection对象。
1: public class RouteDictionary:Dictionary<string,RouteBase>
2: {
3: public RouteData GetRouteData(HttpContextBase httpContext)
4: {
5: foreach (var route in this.Values)
6: {
7: RouteData routeData = route.GetRouteData(httpContext);
8: if (routeData != null)
9: {
10: return routeData;
11: }
12: }
13: return null;
14: }
15: }
UrlRoutingModule
路由表的作用是对当前的HTTP请求的URL进行解析，从而获取一个以Controller和Action名称为核心的路由数据，即上面介绍的RouteData对象。整个解析过程是通过一个类型为UrlRoutingModule的自定义HttpModule来完成的。
1: public class UrlRoutingModule:IHttpModule
2: {
3: public void Dispose()
4: {
5:
6: }
7:
8: public void Init(HttpApplication context)
9: {
10: context.PostResolveRequestCache += OnPostResolveRequestCache;
11: }
12:
13: protected virtual void OnPostResolveRequestCache(object sender, EventArgs e)
14: {
15: HttpContextWrapper httpContext = new HttpContextWrapper(HttpContext.Current);
16: RouteData routeData = RouteTable.Routes.GetRouteData(httpContext);
17: if (routeData == null)
18: {
19: return;
20: }
21: RequestContext requestContext = new RequestContext
22: {
23: RouteData = routeData,
24: HttpContext = httpContext
25: };
26: IHttpHandler handler = routeData.RouteHandler.GetHttpHandler(requestContext);
27: httpContext.RemapHandler(handler);
28: }
29: }
当PostResolveRequestCache事件触发之后，UrlRoutingModule通过RouteTable的静态子都属性Routes得到表示全局路由表的RouteDictionary对象，然后调用其GetRouteData方法并传入用于封装当前HttpContext的HttpContextWrapper对象，最终得到一个封装路由数据的RouteData对象。然后根据该对象和之前得到的HttpContextWrapper对象创建一个表示当前请求上下文的RequestContext对象并将其作为参数传入RouteData和RouteHandler的GetHttpHandler方法中，得到一个HttpContext对象，最后我们调用HttpContextWrapper对象的RemapHandler将Handler重新映射，使之用于对当前HTTP请求进行处理。
本学习内容和代码来自《ASP.NET MVC 4 框架揭秘》